1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, and a two-component type developer for developing an electrostatic image which contains the toner and a carrier.
2. Related Background Art
In electrophotography, a large number of methods are conventionally known, as disclosed, for example, in U.S. Pat. No. 2,297,691. In general, in electrophotography, an electrostatic latent image is formed on a photosensitive member utilizing a photoconductive material and according to various means. Subsequently, the latent image is developed using a toner to form a toner image. Then the toner image is transferred to a recording medium such as paper if necessary, followed by fixing by the action of heat and/or pressure or solvent vapor. A copy is thus obtained. As methods of carrying out development using a toner or methods of fixing toner images, various means have been hitherto proposed, and means suited for their image forming processes have been employed. In recent years, electrophotography is required to achieve higher-speed copying and higher image quality.
Toners are commonly known to be produced by a process comprising melt-mixing or dispersion-mixing additives such as a dye or pigment in a thermoplastic resin to effects uniform dispersion, followed by pulverization and classification to produced a toner having the desired particle diameter.
The toners obtained by such pulverization are usually amorphous and hence have had a limit in achievement of the reproduction faithful to the latent image, which is disadvantageous for making image quality higher. In order to achieve a higher image quality in the toners produced by pulverization, toners must be pulverized to have smaller particle diameters. Making the particle diameters smaller, however, has been problematic in that it requires a greater energy and results in a poor toner yield.
On the other hand, toners obtained by suspension polymerization (hereinafter "polymerization toner(s)") can be free from the above disadvantages. In addition, they can allow a wax to be internally held and hence make it possible to achieve good fixing performance and anti-offset properties.
It, however, has become clear that, when particle diameters are made smaller in the polymerization toners, electrostatic charges suitable for a high image quality can not be readily obtained in a stable quantity. Since the polymerization toners are obtained by granulation in an aqueous medium, polar materials such as a charge control agent localize at particle surfaces. This phenomenon may give an excess charge even when a charge control agent is added in a smaller quantity, and use of the charge control agent in a much smaller quantity brings about the problem that the rise of charge becomes slow. Use of a charge control agent having weak charge-providing properties also tends to bring about the problem that the rise of charge becomes slow.
Japanese Patent Application Laid-open No. 60-238846 discloses a toner containing a saturated polyester. It is true that a toner containing a polyester resin can achieve a sharp particle size distribution. In the toner performances such as charge characteristics, however, unsatisfactory results may be obtained in respect of the speed of rise of charge and the stability in quantity of electrostatic charges over a long period of time.